The present invention relates to data storage devices and, in particular, to a method of encoding servo information on discs used in a hard disc drive.
A typical disc drive storage system includes one or more magnetic discs which are mounted for co-rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic bearing which flies above each magnetic disc. The transducer and the hydrodynamic bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs. An electromechanical actuator operates within a negative feedback, closed-loop servo system to move the data head radially or linearly over the disc surface for track seek operations and holds the transducer directly above a desired track or cylinder on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic discs representing the data to be stored. In retrieving data from the discs, the drive controller controls the electromechanical actuator so that the data head flies above the desired track or cylinder on the magnetic discs, sensing the flux reversals on the magnetic discs, and generating a read signal based on those flux reversals.
In an embedded servo-type system, servo information or (servo bursts) is recorded on data tracks which also contain stored data. The servo bursts are typically temporally spaced evenly about the circumference of each data track. Data is recorded on the data tracks between the servo bursts. In a dedicated servo-type system, an entire disc surface in a disc drive is dedicated to storing the servo information.
As the data head reads the servo information, the transducer provides a position signal which is decoded by a position demodulator and presented in digital form to a servo control processor. The servo control processor essentially compares the relative position error to the track center and commands the actuator to move in order to minimize position error.
Generally, the servo information is written on the disc surfaces during manufacture of the disc assembly. Each disc assembly is mounted to a servo writer support assembly which precisely locates the disc surfaces relative to a reference or origin. The servo writer support assembly supports a position sensor, such as laser light interferometer, which detects the position of the actuator relative to the disc surfaces. The position sensor is electrically inserted within the disc drive""s negative feedback, closed-loop servo system for providing position information to the servo system while the servo data is being written to the disc surfaces. The servo writer support assembly may also support a clock writer transducer which writes a clock pattern onto the disc surface which is used for temporally spacing the servo data about the circumference of each track.
Using a servo writer support assembly to write servo information typically requires many minutes for each disc assembly. Such time slows manufacturing throughput and potentially increases the cost of the finished product. Another limitation is that the servo patterns generally consume about 5% to 10% of the usable recording area in a given disc drive. Further, it is difficult to align the product head skew to the orientation of the pre-written servo patterns. Further still, it is also difficult to align the discs such that the tracks are circular and centered at the spindle rotating center. Yet another limitation is due to the difficulty of aligning the radial positions of the product heads from surface-to-surface-to produce cylinders.
Another known technique for writing servo information uses the disc drive itself to write the servo information in situ. In Situ recording means that the servo patterns are recorded on a fully assembled drive by controlling the position of the actuator arm using the product actuator with feedback from previously written tracks and recording the servo information with the product head. However, one limitation that has generally limited self-servowriting techniques is that disturbances inherent in the drive itself, such as disc flutter and spindle non-repeatable runout (NRRO), limit the radial and circumferencial accuracy of the position sensing pattern.
As industry continues to press disc drive manufacturers to provide disc drives with increased storage capacity at lower costs, it is becoming increasingly important to provide disc drives with high track densities while minimizing manufacturing time and labor costs.
The present invention addresses these and other problems, and offers other advantages over the prior art.
A method of writing servo information for hard disc drives is disclosed. The method includes writing position reference information on a reference disc surface prior to mounting a disc assembly into a hard disc drive. In some embodiments, the position reference information is written at a frequency that is lower than about 5 MHz. In other embodiments, the drive servo frequency can be used. Once the disc assembly is mounted in the disc drive, servo information is written onto the disc surfaces based upon the pre-recorded position reference information. Subsequently, the previously recorded position reference information can be erased or written over. These and other features and benefits of the present invention will become apparent upon reading the following detailed description and review of the associated drawings.